Field
This disclosure relates to a method of cooling a lamp, which is a light source of a projecting type image display apparatus such as a liquid crystal projector or the like when the lamp is turned off.
Description of Related Art
In recent years, demand for a projecting type image display apparatus represented by a liquid crystal projector is rapidly increased. Among others, a tendency toward a high luminance is in rapid progress.
An ultra-high pressure mercury lamp is mainly used as a light source of the projecting type image display apparatus, and the ultra-high pressure mercury lamp generally has electric power of several hundred watts and a gap between electrodes of 1 mm or smaller. Therefore, a current of several amperes flows between the electrodes, and electrons emitted therefrom collide with the electrodes, so that the electrodes get overheated. Consequently, the temperature of distal ends of the electrodes rises to a level on the order or 3000° C., and the temperature in the interior of a bulb formed of quartz glass exceeds 1000° C. The pressure in the interior of the quartz glass is increased to 20 MPa (200 atmospheric pressure) or higher. Therefore, it is important to cool the temperature of the interior of the bulb to an adequate temperature by flowing a cold air thereon from the outside.
In addition, since a heat source of several hundred watts which has been supplied to the ultra-high pressure mercury lamp when the lamp is on becomes zero when the lamp is turned off, the temperature of the quartz glass bulb is rapidly lowered. At this time, a temperature difference between an inner surface and an outer surface of the bulb causes a difference in time of arrival at a point of distortion of the quartz glass, whereby a distortion is generated within the quartz glass. In order to reduce the distortion, a decrease in speed of reduction in temperature of the quartz glass after the lamp is turned off is considered to be effective.
Examples of the decrease in speed of reduction in temperature after the lamp has turned off are disclosed in U.S. Pat. No. 6,788,009 and Japanese Patent No. 4081684.
In U.S. Pat. No. 6,788,009, a configuration in which the lamp is turned off in such a manner that electric power of a lamp is lowered to an extent that an arc discharge is not disappeared (½ to 1/20) when the lamp is turned off, then is maintained for a certain period, and then is turned off is disclosed.
The configuration disclosed in U.S. Pat. No. 6,788,009 produces an effect for making a cooling gradient of the bulb gentle when the lamp is turned off. However, by lowering electric power significantly, adhesion of electrode materials to a bulb tube wall or extinction of the lamp may occur, and hence there is a probability of occurrence of another problem that a lifetime of an ultra-high pressure mercury lamp is shortened.
Japanese Patent No. 4081684 discloses a configuration in which a fan that cools the lamp is stopped when the lamp is turned off.
In Japanese Patent No. 4081684, even though a cooling fan is stopped and a decrease in cooling speed is achieved when the lamp is turned off, the calorific power is significantly reduced (disappeared) by turning off of the lamp, and hence the cooling speed cannot be decreased so much.